1. Field of the Invention
This invention relates to a seismic isolator which comprises vibrating units arranged in a multiplexed manner around a core member suspended from an upper structure, and which is adapted to cut off horizontal vibration of a lower structure.
2. Description of the Related Art
A seismic isolator which comprises a core member suspended from an upper structure, and vibrating units arranged in a multiplexed manner around the core member, and which is adapted to cut of f horizontal vibration of a lower structure, as disclosed in Japanese Patent Publication Nos. 16330/1979, 40842/1979, 574/1988 and 21780/1988 is characterized in that a plurality of frustoconical vibrating units of different scales are arranged in a multiplexed manner with adjacent vibrating units combined with one another by suspension members, a vertical load of the upper structure being borne as a compressive force by the core member and vibrating units, and as a tensile force by suspension members provided among the vibrating units, whereby the upper structure is substantially supported in a suspended state on the lower structure.
Owing to a plurality of vibrating units arranged in a multiplexed manner, a pendulum length larger than a distance between the upper and lower structures is secured. Since a vibration cycle corresponding to the pendulum length equal to the sum of a total length of all of the suspension members is given to the upper structure, the vibration cycle of the upper structure increases to a high level to cut off horizontal vibration of the lower structure.
The vibrating units are formed cylindrically, and the compressive force bearing portions of the vibrating units are formed of face bars. Therefore, in order that the vibrating units positioned relatively on the outer circumferential side support the inner circumferential side vibration units without hindering the relative displacement of the latter vibration units, it is necessary that the vibrating units on the outer circumferential side be larger than those on the inner circumferential side. Therefore, it is necessary that the radius of the vibrating units positioned on the outermost circumferential side be so large that corresponds to the sum of the amplitude of vibrating units, and the area occupied by a seismic isolator generally tends to increase.